USB input vs i-pod input?

I'm looking at new HUs, the two units that are looking the best to me are the Pioneer 5900ib and 6900ub. Slightly different cosmetically, biggest functional difference I can find is that the 5900 has an i-pod input, the 6900 has a USB input. Both inputs work with the i-pod, but what else can the USB do? MP3? The site mentions flash drives, wth could I do with those?

Any other HUs in the $200-250 range that are better IYO than the Pioneers??